The micro-LED technology refers to the LED array of small size integrated on a substrate with high density. Currently, the micro-LED technology is starting development, and it is expected in the industry that a high-quality micro-LED product comes into the market. High-quality micro-LEDs will have a deep affection on the conventional display products such as LCD/OLED that have already been put into the market.
In the prior art, a micro-LED arranged on a display device such as a display panel can have a vertical structure to achieve a high definition. In such a micro-LED, a top electrode is arranged on top of the micro-LED and contacts the top surface of the micro-LED to form a common electrode. The micro-LED emits light from the top surface. The top electrode is transparent.
For example, U.S. Pat. No. 9,367,094 B2 discloses a display module and system application, which is hereby incorporated herein by reference.
Generally, the top electrode on a display substrate is a cathode or N electrode (N metal) and is in an Ohmic contact with the n-EPI surface of the micro-LED. The n-EPI region shall be etched deeply to reach the heavy doping region of n-EPI (e.g., N++ GaN region). This deep etching will add processing complexity. As a result, a cost/yield/reliability issues will rise. The bottom electrode is a anode or P electrode (P metal).
FIG. 1 shows a schematic diagram of vertical micro-LEDs on a growth substrate of the prior art. As shown in FIG. 1, the vertical micro-LEDs 100 are formed on the growth substrate 101. For example, the vertical micro-LEDs 100 are formed through epitaxy growth. The vertical micro-LED 100 may include a P doped region 106, a multiple quantum well (MQW) region 105 and an N doped region 104. The N doped region 104 may include an N++ heavily doped region 103. Optionally, vertical micro-LED 100 may further include an un-doped epitaxy region 102 grown on the growth substrate 101. A P electrode or anode 107 may be formed on the top of the vertical micro-LED 100.
In addition, the conductive top electrode will degrade optical efficiency, to an extend of around 5˜20%, for example, due to its imperfect optical transmittance.
In addition, in such a micro-LED, a side light leakage may decrease the optical efficiency thereof. This may become even more serious when a micro-LED has a small size, for example, equal to or less than 10 μm, for a high-resolution display.
In addition, in such a micro-LED, heat dissipation is limited to the bottom and top surfaces thereof. This is insufficient in a small micro-LED for a high resolution. This may cause an overheating issue. Accordingly, the efficiency and/or lifetime of the micro-LED may degrade.
In addition, in the prior art, the cathode or N electrode are formed after the micro-LEDs are transferred to the display substrate, and thus the processing of the N electrode may be limited.
Therefore, there is a demand in the art that a new solution for a micro-LED of vertical structure shall be proposed to address at least one of the problems in the prior art.